In general, illuminance of a car light may be reduced due to the following causes:
(1) adhesion and accumulation of snow on the outer circumferential surface of the front cover,
(2) adhesion and freezing of rain water or car wash water on the outer circumferential surface of the front cover, and
(3) progression of (1) and (2) due to use of an HID lamp light source having a high light intensity even under a low power consumption (a small heat generation amount).
Structures described in Japanese Laid-Open Patent Publication Nos. 2007-026989 and 10-289602 have been proposed in view of preventing the above illuminance reduction of the car light.
The structure described in Japanese Laid-Open Patent Publication No. 2007-026989 is obtained by printing a conductive pattern on a transparent insulating sheet to prepare a heat generator, and by attaching the heat generator to a formed lens using an in-mold method. Specifically, the conductive pattern in the heat generator is composed of a composition containing a noble metal powder and a solvent-soluble thermoplastic resin.
The structure described in Japanese Laid-Open Patent Publication No. 10-289602 is obtained by attaching a heat generator into a lens portion of a car lamp. The lens portion is heated by applying an electric power to the heat generator under a predetermined condition. The document describes that the heat generator comprises a transparent conductive film of ITO (Indium Tin Oxide), etc.